Kodak IN-1

Wall’s 1924 handbook calls this probably the most generally used of all intensifiers. Kodak printed it in Rochester in 1928 and reprinted it unchanged in London in 1949, and it is built on a salt the same primer describes as virulently poisonous. Both of those things are true at once, and understanding why is most of what this page is for.

☠Level D: this page is not a procedure

Mercury(II) chloride and potassium cyanide are both classified for historical study only, and the Level D policy governs what this page may contain. It may give the chemistry, the historical workflow in outline, the image characteristics and the hazards. It may not give a procedure, and it does not: no mixing sequence, no blackening routine, no time and no temperature appear below as instructions, and the two preparation directions the sources print — one for the bleach, one for the Monckhoven bath — are deliberately not reproduced.

The composition is published because a formula that cannot be read cannot be identified, and identification is the whole use a reader has for this entry. The course asks nobody to weigh any of it. There is no protective equipment list that makes a mercuric chloride bleach and a cyanide blackener acceptable in a domestic darkroom, and none is offered.

The intensifier this course can publish is IN-5, on the facing page of the same handbook.

Bleaching solution — the mercury bleach

Ingredient	Quantity	Form the source specifies
Potassium bromide	22.5 g
Mercury(II) chloride	22.5 g	mercuric chloride, corrosive sublimate
Water	to make 1000 mL	2.25 per cent w/v of each salt. No temperature is published for making it up. Wall records that the mercuric halide solution is sensitive to light and was kept in the dark.
Kodak Limited prints this bath without a label, as "the following solution"; the name here is descriptive. Its equal weights of two very different substances are a coincidence of the formula rather than a stoichiometry.

The Monckhoven blackening solution — the high-contrast blackener

Ingredient	Quantity	Form the source specifies
Potassium cyanide	15 g or 15 g (sodium cyanide, offered at the same weight)	sodium or potassium cyanide; the handbook offers either
Silver nitrate	22.5 g	crystals
Water	to make 1000 mL	Both printings give the same make-up volume. Kodak's preparation direction is not reproduced on this page; see Mixing.
The 1928 primer names this Monckhoven's Intensifier and records that it was used chiefly for negatives made by photo-engravers. It is the only one of the four blackening treatments the handbook prints as a composition, and it is the one that contains a cyanide.

Used in this order — the historical high-contrast treatment, as both printings set it out

1. Bleaching solution — until the negative is white — A criterion and not a time; neither printing gives one. Wall's period practice judged the end point by looking at the plate from the back.
2. Water — wash thoroughly — Kodak's own instruction between the two baths. Wall records that the mercury salt is held tenaciously by the gelatin and that period practice used an acidulated wash to remove it - a step whose hazard the course reads quite differently from the way 1924 did.
3. The Monckhoven blackening solution — No time is published in either printing. Wall records that a negative left too long in this bath is reduced rather than intensified.

Bleach the negative in the bleaching solution until it is white, then wash thoroughly. Alternatively, to increase contrast greatly, treat with the Monckhoven solution after bleaching.

The word "alternatively" is Kodak's, and what it is an alternative to is the ordinary blackening - a 10 per cent w/v sulfite solution, D-72 diluted 1 + 2, or 10 per cent ammonia, which Kodak says give progressively greater density in that order. Those three are named as strengths rather than printed as formulas, so they are not solutions of this entry; they are described in the page body. This sequence is recorded because it is what the source publishes, not as something to follow.

Purpose

To add density to the densest parts of a line negative and leave the rest alone. Kodak Limited’s header is exact and is worth reading as a specification rather than as a description: “An intensifier for line and process negatives giving increased maximum density with little intensification of intermediate tones.”

That is not what the word intensifier suggests. This one does not lift the whole curve; it lifts the shoulder. For a photo-engraver working from a line original, whose negative should be black and clear and nothing in between, that is exactly the right behaviour — and for a pictorial negative it is exactly the wrong one.

The 1928 primer gives the general principle underneath it: intensification is the opposite of reduction, the object being to increase contrast, and it is done by the deposition of some material on the silver image. The material here is mercury, and then — if the Monckhoven bath is used — silver as well.

Recommended uses

These are the uses the sources record. They are set down as history and not as recommendations; the course recommends none of them.

Line and process negatives, which is Kodak’s own scope for the formula and the one place its shoulder-only behaviour is an advantage.

Photo-engravers’ negatives, specifically, for the Monckhoven variant. The 1928 primer records that this very powerful method was used chiefly for negatives made by photo-engravers, and that the cyanide cuts the shadows very slightly at the same time as the highlights are intensified, so that a great increase in contrast is obtained.

Where a great deal of contrast was wanted and permanence was not the point. Kodak Limited says in as many words that where permanence of the resulting image is essential, ammonia should not be used for blackening; the 1928 primer goes further and records that mercury-intensified images are not as stable as chromium-intensified ones.

Nothing in a modern darkroom. No part of this course teaches negative intensification as practice, and if it did it would not teach this one.

When another formula is preferable

• Kodak IN-5, the silver intensifier, printed two pages later in the same handbook. It is the only intensifier of Kodak Limited’s five whose every reagent has a written encyclopaedia entry at Level A or B, and it is the alternative this course can publish as a formula to mix. It is proportional rather than shoulder-only, so it is not a substitute for IN-1’s particular effect — it is a different tool that happens to be available.
• The 1928 primer’s re-development route, which it calls perhaps the simplest method of intensification: the rehalogenating ferricyanide-bromide bleach followed by the sulfide bath of T-52. Both baths are published in this formulary; the primer gives the pairing no formula number and no quantities of its own, so it has no entry.
• IN-4 and IN-21 are not alternatives, whatever the 1928 primer’s preference for chromium on permanence grounds. Both are Level D in this course for reasons of their own.
• A better negative. The course’s position on a thin negative is that the answer is exposure and development, and that no intensifier puts detail into a shadow that never had any.

Mixing

This section gives no mixing procedure, and the omission is the point.

Both sources print a preparation direction for the Monckhoven bath — a sequence involving dissolving two substances separately, combining them in a stated direction until a described end point is reached, standing and filtering. It is not reproduced here. It is a procedure for preparing a cyanide solution, and the Level D policy is explicit that a partial procedure is more dangerous than none because it reads as permission and omits the controls.

What can be said without giving instructions is what the shape of the historical workflow was, because that is what makes the chemistry legible:

It was a two-stage process. The negative was bleached white in the mercury bath, washed, and then blackened in a second bath. Nothing about the first stage settled the result; the choice of second bath did.

There were four blackeners and they made a ladder. Kodak Limited lists a 10 per cent w/v sulfite solution, a developing solution such as D-72 diluted 1 + 2, and 10 per cent ammonia, “these giving progressively greater density in the order given”, with the Monckhoven silver-cyanide bath as the alternative for a great increase in contrast. Wall’s 1924 handbook lists ten blackening reagents and puts numbers on the ladder: taking the original density as 1, several of them gave about 2 and ammonia about 2.5. The three Kodak names as strengths are not formulas and are not published here as solutions.

The washing between the baths was the difficult part. Wall records that the mercury salt is held tenaciously by the gelatin, that an ordinary wash does not remove it, that period practice used an acidulated wash of hydrochloric acid at 1 part in 300 through several changes, and that stains follow if it is not removed. Read against this course’s incompatibility matrix, that is an acid wash on a plate about to enter a cyanide bath, and it is a good illustration of why an outline is as far as this page goes.

Behaviour

Recorded from the sources so that the formula can be understood, not so that it can be run.

No time and no temperature are published for any stage, in either printing. The end point of the bleach is a criterion — the negative is white — and Wall’s practice judged it by looking at the plate from the back and was emphatic that the action should not be stopped before that point.

The result was chosen after the bleach, not before it. This is the property that makes the formula interesting: the bleach converts and commits nothing, and the blackener decides how much density comes back. Kodak’s ordered list and Wall’s numbers are two accounts of the same ladder.

It could go backwards. Wall records that a negative left too long in the Monckhoven bath is reduced rather than intensified.

The plate had to be free of hypo before it went in, per Wall, which puts a thorough wash in front of the whole sequence.

No capacity and no keeping figure is published for either bath. Wall records only that the mercuric halide solution is sensitive to light and was kept in the dark.

Image characteristics

The shoulder rises and the mid-tones barely move. That is Kodak Limited’s own header, and it is the sentence that decides which negatives the formula suited.

With the Monckhoven bath the shadows are cut as well, which is why the 1928 primer describes the result as a great increase in contrast rather than an increase in density: the top goes up and the bottom comes down.

How much density came back depended on the blackener, on a ladder both sources give: sulfite lowest, a developing solution next, ammonia highest at about two and a half times the original density by Wall’s reckoning.

The image was no longer made of silver alone. After redevelopment, in the 1928 primer’s account, every part of silver has an equal part of mercury beside it. That is the whole mechanism and it is also the permanence problem.

It does not last. The 1928 primer’s finding is that mercury-intensified images are not as stable as chromium-intensified ones, and Kodak Limited’s own instruction is that ammonia should not be used for blackening where permanence of the resulting image is essential. A negative that was intensified this way in 1930 has had ninety years to demonstrate it.

It leaves a signature. The Getty Conservation Institute’s atlas, quoted on the mercury(II) chloride page for a different process, records that mercury remains detectable by X-ray fluorescence long afterwards. An intensified negative in a collection can be identified as one.

The mechanism

The bleach is a displacement and the blackening is a reduction. The mercury(II) chloride page carries the bleaching reaction with its source and this page does not restate the chemistry that belongs there. What it can add is what the formula contributes.

⚗Why the density is added rather than rearranged

The 1928 primer’s account of the bleach is that a silver image placed in a solution of mercuric chloride forms a mixture of mercurous chloride and silver chloride, and that the bleached image appears white.

2 Ag + 2 HgCl₂ → 2 AgCl + Hg₂Cl₂

The bleach, as the 1928 primer states it: two white salts where the silver stood

Nothing has left the negative at this point and nothing has been added to its density; two white solids are sitting where a black image was. What happens next is the intensification. If the white image is redeveloped, both chlorides go back to metal, so that in the primer’s words every part of silver now has an equal part of mercury added to it — and that added mercury is material the negative did not contain before. That is the difference between an intensifier and a developer.

Blackening with ammonia instead gives, per the primer, a black mercury ammonium chloride and a higher degree of intensification still. No equation is printed here for that product, because the corpus names the compound and does not establish its formula, and a plausible equation for a named compound is exactly what Rule 1 forbids.

What the bromide is doing, and what the corpus does not say it is doing. Wall states the purpose plainly: salt, ammonium chloride, hydrochloric acid or potassium bromide were added to a mercuric chloride bleach to increase the solubility of the mercury salt, and the bromide gave the greater increase of density. Kodak’s formula is the bromide version. What no source in this corpus says is what the bromide does to the identity of the bleached image — whether the silver comes down as the chloride the primer names, or as the bromide the bath also offers it, or as both. This page does not decide it, and the reaction above is printed as the primer’s account of a chloride bath rather than as a full account of this one.

Why the Monckhoven bath increases contrast rather than density. It carries silver, dissolved by cyanide, so it deposits silver on the bleached image as well as reducing the mercury salts — and the same cyanide is a silver solvent, which the primer says cuts the shadows very slightly at the same time. Building the top of the scale and dissolving the bottom of it are two ways of doing the same thing to a curve.

Function of every ingredient

Mercury(II) chloride, 22.5 g to the litre of the bleaching solution. The intensifier proper: the substance that converts the image and then, after blackening, adds mercury to every part of it. The 1928 primer’s remark about it is worth keeping in one piece — mercury bichloride is a virulently poisonous salt, sometimes known popularly as corrosive sublimate, and its only use in photography is for intensification. There is no other job it does in a darkroom, which is why removing it from a course removes nothing else. It is soluble enough for practical use where most mercuric salts are not, which is why this is the mercury salt the formula names. Its page carries the classification — signal word Danger, fatal if swallowed and fatal in contact with skin, in every notification — and the exposure limit, and this page does not restate or soften either. More or less of it is not a control the sources discuss, because the bleach is taken to a visible end point rather than to a time.

Potassium bromide, 22.5 g to the litre of the bleaching solution. The solubility aid, and the reason this bath is the version it is. Wall names four substances that were added to a mercuric chloride bleach for that purpose — common salt, ammonium chloride, hydrochloric acid and the bromide — and records that the bromide gives the greater increase of density. So it is not an inert helper: the choice among four solubility aids changed the result, and Kodak’s formula chose the one that gave most. More of it is not a control either source publishes. Note the coincidence in the formula and do not read anything into it: the bromide and the mercuric chloride are at the same 22.5 g, which is a round number in the avoirdupois column rather than a stoichiometric relation between two substances of quite different relative molecular mass. It is the one Level A ingredient on this page, and it is the reason a reader should never judge a formula’s hazard from the friendliest thing in it.

Potassium cyanide — or sodium cyanide — 15.0 g to the litre of the Monckhoven blackening solution. The silver solvent, and the ingredient that makes the Monckhoven bath both possible and unusable. Its job is to hold silver in solution as a complex so that it can be deposited on the bleached image, and its side effect is the one the 1928 primer names: the cyanide cuts the shadows very slightly while the highlights build, which is where the great increase in contrast comes from. Two things are worth stating about the printed line. First, both printings offer sodium or potassium cyanide at the same 15.0 g, and the two salts have different relative molecular masses, so they are not equivalent gram for gram; neither source remarks on it and this page does not resolve it. Second, this is the ingredient whose presence Kodak Limited itself flagged, in a warning printed under the formula in 1949 and reproduced in substance under Safety below. The sodium cyanide page carries the second salt’s own record.

Silver nitrate, 22.5 g to the litre of the Monckhoven blackening solution. The silver that this bath adds on top of the mercury — the reason Monckhoven’s variant gives more than a simple redevelopment does. It is the same substance and the same role as in IN-5, where it does the whole job on its own without any mercury or cyanide anywhere near it, which is the neatest possible statement of what this course keeps and what it does not. It is Level B on its own; it is not what makes this page Level D.

Water, to make each bath up to its litre. Both printings state make-up volumes rather than added volumes, so both strengths are fixed. No temperature is published for either.

Interactions

With acid, which is the interaction that governs everything else. Potassium cyanide releases hydrogen cyanide gas on contact with any acid, and a darkroom is a room with an acid stop bath in it. Kodak Limited printed that fact in its own warning under this formula in 1949. The incompatibility matrix carries the pair.

With the acid wash the period practice used. Wall’s method for removing the tenaciously held mercury salt was an acidulated wash, and the plate then went into a blackening bath which might be the cyanide one. Two sound period instructions, each defensible on its own, that put an acid and a cyanide in the same sequence.

With gelatin, which holds the mercury salt rather than releasing it — CAMEO lists gelatin and albumin among mercury(II) chloride’s incompatibilities, which is the same fact Wall recorded from the darkroom.

With sulfites, sulfides and ammonia, all of which CAMEO lists as incompatible with mercury(II) chloride and several of which are the reagents the historical practice used to blacken the image. As the chemical page puts it, the second bath was an intended incompatibility.

With hypo, which had to be washed out of the plate first.

With light, which the mercuric halide solution is sensitive to, per Wall.

Variants

The 1928 Eastman Kodak printing is not a variant: it is the same formula at the same quantities, and it is recorded as corroboration. What the primer adds is the attribution — the silver-cyanide bath is Monckhoven’s — and the finding about permanence.

The four blackening treatments are the real variation in this formula, and Kodak Limited presents them as a ladder: a 10 per cent w/v sulfite solution, D-72 diluted 1 + 2, or 10 per cent ammonia, giving progressively greater density in that order, with the Monckhoven bath as the alternative for a great increase in contrast. Wall’s list runs to ten, including a ferrous oxalate developer, amidol, sodium sulphide, Schlippe’s salt and a stannous tartrate, with a note against several of them that they give too much intensification for ordinary work.

Wall’s own bleach, which is a different formula: mercuric chloride at 20 g/L with one of four solubility aids at 20 g/L, and a second version carrying a large proportion of ammonium chloride which he says is meant to facilitate the removal of the mercury salt. It is recorded here as evidence that Kodak’s bromide version was one choice among several, not as a formula.

The Eastman Intensifier, which the 1928 primer names as a single-solution product sold ready made, with which the intensification proceeds continuously so that it can be stopped at any time, and which the primer says gives less intensification than the two-solution method but is far more convenient. Its composition is not disclosed, so it can be recorded here only as behaviour: a bought bottle, a continuous action, a lower ceiling. No reconstruction of it appears anywhere in this course.

No course variant is offered and none could be. There is no weaker mercury bath and no substitution that changes what this formula is. The safer route to a similar end is a different formula with different chemistry, and it is IN-5.

Safety

Level D, from mercury(II) chloride and potassium cyanide — two of the families the Level D policy names, in one formula. Read those pages and the policy; this one does not restate their classifications, their exposure limits or their first aid, and does not soften them.

Why this is Level D and not Level C, in the rubric’s own terms: Level C is the level at which a fume cupboard, controlled waste and a second competent person make a procedure acceptable, and the course gives the procedure to a reader who has those. Here the failure modes are a salt that is fatal by ingestion and by skin contact, and a salt that gives off a lethal gas on contact with any acid, in a room that contains an acid stop bath by design. No set of controls a domestic reader can assemble makes that reasonable, so no procedure is given to anyone.

☠What Kodak Limited itself printed under this formula in 1949, and what has changed since

The handbook’s own warning is unusually blunt for a formulary and deserves to be read as written: cyanide is a deadly poison and should be handled with extreme care; it reacts with acid to form poisonous hydrogen cyanide gas; and cyanide solutions should never be used in poorly ventilated rooms. Three sentences, all still true.

The fourth sentence is the one that has not survived. For discarding a solution containing cyanide, Kodak Limited’s 1949 instruction was to run water and flush it out of the sink quickly. That is not what this course says and it is not what any current guidance says. It is recorded because it shows what “handled with extreme care” meant in 1949, and because a reader working from a period manual will meet it. See Waste.

Wall’s 1924 handbook contains a claim of the same vintage about the mercury salt: that absorption through the skin, even through cuts and abrasions, was of practically no consequence. That claim is wrong. The notified classification for mercury(II) chloride carries H310, fatal in contact with skin, in every one of 218 reports. This is the clearest example in the formulary of why a period source establishes what was done and never what is safe.

The salt is not the whole of the mercury hazard. The Level D policy makes the point about the daguerreotype and it applies to any mercury operation: what puts mercury processes out of reach is partly that the metal has a vapour pressure at room temperature, the vapour is invisible and odourless, and EH40 sets mercury and its divalent inorganic compounds at 0.02 mg/m³.

Nothing on this page is a control and none is offered. There is no glove specification, no ventilation rate and no waste procedure below that would make this reasonable, because the classification is not a statement about how hard the controls would be. It is a statement that a page cannot assume them.

Storage

What the sources record, as history.

The mercuric halide solution was kept in the dark, per Wall, because it is sensitive to light.

Neither printing publishes a keeping figure for either bath, and none is invented here.

Nothing on this page is stocked in a home darkroom. The mercury(II) chloride page’s storage entry says so for the substance, and this formula does not create an exception.

Where such a solution exists in a collection or a laboratory, it is the institution’s own procedures that govern it, not a course page. A labelled historical bottle of corrosive sublimate found in an old darkroom is a hazardous waste question for a professional and not a find.

Incompatibilities

Acids of every kind, against the cyanide. Hydrogen cyanide. This is the incompatibility that governs the formula from the bottle to the sink.

Gelatin and albumin, against mercury(II) chloride, per CAMEO — the reason the mercury salt is held in a bleached plate and does not simply wash out.

Alkalis, ammonia, lime water, carbonates, borax and phosphates, per CAMEO. The ammonia reaction is the one the historical practice used deliberately.

Sulfites, sulfides, formates and hypophosphites, per CAMEO, several of them the blackening reagents.

Bromides and silver, lead, iron and copper salts, per CAMEO — which includes two of the formula’s own ingredients, and is a reminder that an incompatibility list describes reactions rather than forbidding them.

Sodium and potassium metal, with which CAMEO records that mixtures are shock-sensitive explosives.

See incompatibilities.

Waste

This is the section where the historical source is most obviously superseded, and it is worth being explicit about it.

Kodak Limited’s 1949 instruction for a spent cyanide solution was to run water and flush it out of the sink quickly. This course does not repeat that instruction and does not endorse it. A cyanide-bearing and mercury-bearing waste is a hazardous waste in every jurisdiction the course has looked at, and the sewer is closed to it — not on a threshold but absolutely.

Nothing here is generated in this course, because the process is not carried out. The mercury(II) chloride and potassium cyanide pages carry the waste position for the substances themselves.

If such a solution exists, from a historical stock or an institutional process, it is a matter for a licensed hazardous-waste route and a professional, and the disposal ruling explains why the course will not go further than that. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

There is nothing to troubleshoot, because there is no procedure. What follows is what the sources record going wrong, kept because it explains the chemistry and because it is what a reader will meet in a period manual.

“The bleach was stopped too soon.” Wall is emphatic that the action should not be stopped before the image seen from the back is white, and the reason is chemical rather than aesthetic: a partly bleached image is a mixture of untouched silver and converted salt, and the blackening then works on part of the negative only.

“Stains appeared later.” Wall attributes them to mercury salt left in the gelatin, which an ordinary wash does not remove. It is the same fact CAMEO lists as an incompatibility with gelatin.

“The negative came out weaker.” Wall records this specifically of the Monckhoven bath: a negative left in it too long is reduced rather than intensified, because the cyanide is a silver solvent as well as a silver carrier.

“The intensification faded.” Expected, over decades. The 1928 primer’s finding is that mercury-intensified images are less stable than chromium-intensified ones, and Kodak Limited’s own advice was to avoid the ammonia blackener where permanence mattered.

“There is an old bottle of this in the darkroom I inherited.” That is not a troubleshooting question. It is a hazardous-waste question, and the answer is a professional.

Experiments

No experiment on this page involves making or using this formula. These are things a reader can do that answer the questions the formula raises.

Identify one. Negatives intensified this way exist in collections, and the Getty atlas records that mercury remains detectable by X-ray fluorescence long after treatment. Where a collection publishes its analytical results, read them against this formula’s composition: the presence of mercury with silver, and the shoulder-heavy density distribution Kodak’s header describes, are two independent signatures of the same treatment.

Read the ladder as a claim. Kodak Limited’s ordering — sulfite, then a developing solution, then ammonia, giving progressively greater density — and Wall’s numbers, about 2 for several blackeners and about 2.5 for ammonia, are two sources making a quantitative claim about the same process. Set them side by side and note what neither of them says: on what material, at what original density, and measured how.

Do the substitution on paper. Take the shoulder-only behaviour Kodak’s header describes and draw it on a characteristic curve. Then draw what IN-5’s proportional intensification would do to the same negative. The two are different operations, and seeing that on paper is the reason this course can publish one and not the other.

Assess it yourself, which is Part XXVI’s own assignment. This formula is a good exercise because the answer is unambiguous and the reasoning has several independent strands: the acute classifications, the exposure limits, the incompatibility with an acid that is in the room by design, the period source that got the skin hazard wrong, and the manufacturer’s own disposal advice that no longer stands. Write the assessment before reading Safety above, then compare.